a) Field of the Invention
The present invention relates to a rotary polygon mirror optical scanning device having a hydrodynamic bearing motor, and more specifically, a rotary polygon mirror optical scanning device capable of rotating a rotary polygon mirror without accompanying surface vibrations.
b) Description of the Related Art
Generally, in rotary polygon mirror optical scanning devices having a hydrodynamic bearing motor, a rotary polygon mirror is mounted on one side edge of a rotor. This easily causes rotational unbalance of the rotor. When the rotor becomes out of balance in rotations causing vibrations in a shaft, the gap becomes varied in a clearance between the rotor and a fixed-side bearing portion which rotatably supports the rotor via a hydrodynamic bearing mechanism. This may cause an improper hydrodynamic pressure generating condition, which degrades bearing performance. Also, when the rotational balance of the rotor is lost, mirror surface vibrations are caused in the rotary polygon mirror mounted on the rotor. This may hinder the rotary polygon mirror from performing excellent optical scanning.
U.S. Pat. No. 5,642,150, for example, discloses a structure to prevent the rotary polygon mirror from having surface vibrations. In this structure, the axial mounting position of the rotary polygon mirror is set as high as one of the hydrodynamic pressure generating grooves which are produced at positions away from one another by a predetermined distance; the mounting position of the rotary polygon mirror which is large in inertial mass in the rotor is positioned at the hydrodynamic pressure generating portion (the portion with high bearing rigidity). Therefore, the rotary polygon mirror is not easily subjected to induced surface vibrations. Surface vibrations are also prevented from occurring by axially setting the rotational torque generating portion in the motor portion as high as the cutting area of the other hydrodynamic pressure generating groove.
In the rotary polygon mirror optical scanning device, however, the diameter of the fixed shaft is small compared to the diameter of the rotary polygon mirror. For this reason, the surface vibrations described above are still generated as far as it substantially affects optical scanning.